The discussion of any prior art documents, techniques, methods or apparatus is not to be taken to constitute any admission or evidence that such prior art forms, or ever formed, part of the common general knowledge.
Pumpable props are elongate hollow members that can be filled (pumped) with a settable material such as grout to form a strong load bearing column. It is known for these props to be length adjustable.
A known type of pumpable prop is a pumpable crib bag, and an example of this is described in U.S. Pat. No. 8,246,276. These props typically comprise a long cylindrical bag, sometimes containing external reinforcement. The bag is extended, fixed to the mine roof and then filled with grout. The US patent acknowledges the expense of existing crib bags and the difficulty in installation. The patent also acknowledges the relatively low load supporting properties of this type of prop and improves upon the prop by including an internal reinforcement which is embedded in the grout. While this can improve the load supporting properties of the pumpable crib bag type prop, it also increases the cost of this type of prop, and may make the filling of the prop more difficult. Bag type props generally have spaced wire hoops or a spiral wire reinforcement to permit collapsibility of the bag.
Another type of suitable prop is described in U.S. Pat. No. 5,273,378 (Merz). This prop is height adjustable and can be filled with a settable grout. The prop is made from steel segments.
The ability of grout filled props to withstand loads is determined by compressing the prop to the point of failure. As the prop is compressed, the grout, at some stage, begins to fracture. As long as the fractured grout is retained within the prop and the prop generally remains upright, the prop will retain a load bearing ability and not fail. This simulates movement in the mine roof. At some stage the prop wall will split causing the fractured grout to spill from the prop and at this stage the load bearing ability of the prop fails. With the bag type columns, it is found that compression of a 750-800 mm diameter 2.5 m high column by less than 200 mm will cause the bag to split, the grout to spill, and the column to fail. Put differently, should a mine roof collapse more than 200 mm, the bag type column will no longer be able to support the mine roof.
Somewhat surprisingly, it is also found that a column made of steel tubes filled with grout is also not very effective. It is found that compression of a 600 mm diameter 2.5 m high steel column by about 300 mm causes the column to fail. While a steel column is much stronger than a bag type column, it is found that compression (that is vertical displacement) of the column causes the column to “kink” which forms a line of weakness in the metal and which causes the metal to split. The kink configuration also results in the column adopting a non-linear configuration which makes bursting of the column much easier. Put differently, a roof collapse of more than 300 mm can cause a grout filled steel column to fail.
Another disadvantage with existing grout filled props is the amount of labor required in the erection. The bag type column requires the extended bag to be secured somehow to the roof of the mine to keep it in the upright condition while being filled with grout. The attachment and removal of the grout filling tool can also be laborious. The metal type column is very heavy and the inventors consider that the metal type column as described in the above US patent (Merz) would require five men and a small crane or forklift truck for erection. Installing a multiplicity of Merz props would be very time consuming and laborious.
A further problem is that it is often difficult and labor intensive to install a prop in a mine. This is because the roof and floor of the mine may be uneven so that it is difficult to maneuver the prop and locate it in its desired position.
Another disadvantage with existing fillable props is the difficulty and time taken to attach and remove the grout filling tube to the prop. Most existing fillable props are filled from the top of the prop. The reason for this is that it enables grout to be used with a faster gel time. A prop filled from the bottom requires grout to have a longer gel time to prevent the grout from setting before the prop is filled. If the mine roof is more than about 2 meters high, a ladder is required to enable a person to attach the grout line to the grout inlet on the prop. When the prop is filled, the person again needs to use a ladder to remove the grout line from the now filled prop. In a muddy, wet and slippery mine, climbing a ladder is hazardous.
Another disadvantage with existing fillable props is the tendency for grout to spew back out of the inlet when the prop is full and the grout filler tube is removed. Some props are fitted with a one way valve but these valves can be quite flimsy and are not entirely reliable. Other props use expensive ball type valves that add to the cost of the prop as the valve usually stays on the prop and is not reused. If grout spews back out of the inlet, a void is created in the column which creates a zone of weakness. There would be an advantage to provide a fillable prop with an inexpensive yet reliable backflow prevention valve.
It is an object of the present invention to address one or more of the above-described problems or to at least provide a useful alternative to those props and stays that have hitherto been known in the prior art.